1. Field of the Invention
The present invention relates generally to combinations of video signals required in a TV production control room thereby notably obtaining special effects such as image inlayings and superimpositions and also cross fades and black fades.
2. Description of the Prior Art
As known, a television control room contains plural video sources transmitting synchronous video signals, in-phase and of the same nature or standard, so as to select certain of them, according to requirements, to create special effects visible in the image represented by the video signal resulting from the combination.
Image mixer devices in analog television at present have a basic architecture shown schematically in FIGS. 1 to 4 appended hereto. All these mixer devices contain a videofrequency signal switching grid GR receiving incoming video signals outgoing from the video sources for selecting, as required, a certain number of these signals to be combined in elementary mixers of "variable gain switch" type CGV.
According to the simple example shown in FIG. 1, the mixer device comprises one single elementary mixer CGV only. In the mixer CGV, two video signals IA and IB selected by the grid GR from the signals IM.sub.1 to IM.sub.J are applied to two variable gain amplifiers or analog multipliers MA and MB also receiving two control signals CA and CB respectively. The two video signals modified by the control signals are then added in an adder AD into a resultant signal IR corresponding to the composite image to be broadcast.
The control signals CA and CB are produced in a control circuit CO, as from the selection of effect and dissolve signals available in the control room so as to obtain in the composite image one of the effects and dissolves required.
To create a composite image resulting from the combination of more than two images, the mixer device must contain several mixers CGV.
For example, as illustrated in FIGS. 2 and 3, the resultant signal IR is deduced from a combination of four incoming video signals IA, IB, IC and ID selected by the grid CR. According to FIG. 2, the mixer device comprises three mixers CGV1, CGV2 and CGV3 connected in cascade, i.e., the mixer CGV1 mixes signals IA and IB into a signal S1, the mixer CGV2 mixes signals S1 and IC into an signal S2, and mixer CGV3 mixes signals S2 and ID into a signal S3=IR. In the mixer device shown in FIG. 3, mixers CGV1 and CGV2 respectively mix the signals IA and IB and the signals IC and ID into signals S1 and S2' which are mixed into a signal S3'=IR in a third CGV3 mixer.
Referring to FIG. 4, the mixer device also includes three mixers CGV1, CGV2 and CGV3 and an auxiliary switch grid GRA whereby the four signals IA, IB, IC and ID can be combined, like in the mixer devices shown in FIGS. 2 and 3, but also whereby certain of these four signals can be combined with other incoming video signals IE, IF and IG selected by the grid GR.
The resultant image in a mixer device is thus designed as a superimposition of several images circumscribed by predetermined contours and contained in superposed planes. FIG. 5 shows an image arrangement resulting from the superimposition of four image planes P1 to P4 designed to inlay a title, a person, a second background and a first background transmitted by video sources selected by the grid GR.
In mixer devices according to the prior art the contour of the resultant image outgoing from a mixer CGV is determined by an effect signal, and the appearance and vanishing of the resultant image are determined by the selected dissolve. The image inside the contour is supplied by the selected video source connected to one of the two inputs of the mixer and the image outside the contour is supplied by the selected video source connected to the other input of the mixer. To compose an image from several images contained in different contours, reflection through plane superposition is not immediate. Such a composition requires at least a switching grid GR which has a wide band of approximately 6 MHz and, therefore which is costly, and requires combinations of elementary mixers CGV which introduce different propagation times for the various selected dissolves of the video signals, and subsequently, phase-shifts between these signals on output from the mixer device are reflected in the resultant image.
Moreover, the pairs of control signals CA and CB in the mixers are independent from each other. Aware that the incoming video signals, but also the resultant video signal are normalized, i.e., have amplitudes lying between predetermined limits, the amplitude of the resultant signal risks overstepping the predetermined limits and must thus be clipped which causes defects in the resultant image. The result is that the surimposition of image planes imposes a sum of incoming signals having crossed through the mixers CGV, lying in the predetermined limits so as to prevent any clipping of the resultant signal: for example, for several effects, the control signals should be subjected to complementarity conditions, so as to avoid an black or white underlining on the chopped and dissolved images.